The catalog pg_opclass defines
index access method operator classes. Each operator class defines
semantics for index columns of a particular data type and a
particular index access method. An operator class essentially
specifies that a particular operator family is applicable to a
particular indexable column data type. The set of operators from
the family that are actually usable with the indexed column are
whichever ones accept the column's data type as their lefthand
input.

An operator class's opcmethod
must match the opfmethod of its
containing operator family. Also, there must be no more than one
pg_opclass row having opcdefault true for any given combination of
opcmethod and opcintype.